Desalination of seawater and treatment of high saline surface waters are important processes for producing potable and irrigation water. However, saline water sources also contain other trace contaminants, like boron, which may appear in the final product water. The main source of boron in brackish surface waters or ground water can be traced to either residuals from waste water treatment plants (mainly borate from detergent formulations), or to leachables from subsurface strata. In seawater sources, the typical boron concentration in the raw water is 4.5 mg/L. Depending on location and seasonal effects, the boron concentration can exceed 7 mg/L (e.g. in the Arabian Gulf). Recent changes in World Health Organization guidelines for boron limits make it attractive for new medium and large membrane desalination plants to have upper limit values for boron in their product water of between 0.3 and 1.0 mg/L. In both seawater and brackish waters, boron is usually present as boric acid, which at higher concentrations and temperatures, form polymers. This behavior is very important in the water cycles in pressurized water reactors.
When removing boron from seawater, treatment plants consider the equilibrium given (Equation 1) below.B(OH)3+OH−B(OH)4−with pKb˜4.8  Equation 1tB=B(OH)3+B(OH)4−  Equation 2
The distribution of the charged and the uncharged species can be calculated, as a function of pOH and pKb. At high pH (larger about 9.2) the charged species prevails and at low and neutral pH, the neutral species is dominant. Mesmer et al. have reported the pKb as a function of temperature and ionic strength using KCl as the method of adjusting ionic strength. (Mesmer, R. E.; Baes, C. F., Jr.; Sweeton, F. H., Inorganic Chemistry (1972), 11(3), 537-43).
Reverse osmosis (RO) technology used in desalination also removes some boron. Ionic behavior is important in determining which species are removed from water during RO. Most membranes have a medium to high rejection of the charged species, and a low rejection of the uncharged species. Different types of membranes are used in reverse osmosis membranes for seawater and brackish water treatment. For commercial seawater membrane elements, rejection of uncharged B(OH)3 varies between 80 and 95%. Brackish water membrane filtration elements reject between only 30 and 80% of the uncharged species. At higher pH values, rejection of boron by separation membranes is significantly improved, due to the shift to the better rejected B(OH)4− species. Rejection of the charged species is 98-99% with brackish water elements, and 99.5% and higher with seawater elements. At lower or neutral pH, treatment plants may use multiple RO stages or passes to decrease the boron level, but at added cost.
In addition to pH, temperature has a strong influence on the pKb value of boron. When temperature increases, pKb drops. A drop in pKb results in a stronger concentration of the B(OH)3 species, which RO membranes reject poorly. Seawater feeds to desalination plants can range between about 10 to 45° C., and can therefore impact RO removal of boron.
Boron removal can be enhanced by replacing one or more RO membrane modules with resin-based boron removal stage. Redondo, et. al describe a system of desalination with filtration membranes combined with resin-based boron removal (Redondo, J., Busch, M. & De Witte, J., “Boron removal from seawater using FILMTEC™ high rejection membranes”, Desalination 156 (2003)).
The performance of boron selective resins (BSRs) is less sensitive to pH and temperature that of than membranes. Currently available, commercial BSRs typically comprise macroporous cross linked poly-styrenic resins, functionalized with N-methyl-D-glucamine (NMG), also called 1-amino-1-deoxy-D-glucitol. FIG. 1 illustrates a structure for N-methyl-D-glucamine. The NMG moieties of BSR capture boron via a covalent chemical reaction and an internal coordination complexation, rather than simple ion exchange. Over a wide range of pH, boric acid “adds” across one of the cis-diol pairs of the functional group to form this relatively stable cis-diol borate ester complex. FIG. 2 illustrates the structure of such an ester complex.
While BSRs may possess as much as 0.9 moles of NMG per Liter of resin volume, their operating capacities for boron are typically somewhat lower. Usable operating capacity depends strongly on the concentration of boron in the feed, the operational flow rate, the efficiency of regeneration, and the outlet boron concentration cut-off limit.
In a typical seawater desalination configuration, a column containing BSR operates downstream of one or more RO membrane elements, so that the feed to the column is permeate from the RO output. Typical seawater reverse osmosis (SWRO) permeate typically contains ˜1.5 mg/L boron. With this permeate as a feed, a resin bed containing BSR may be expected to have a breakthrough limit of 0.10 mg/L boron at the outlet, at flow rate of 30 bed volumes per hour. In this context the typical operating capacity for BSR for boron is between 1.4 to 2.0 g boron per Liter of resin.
In a boron removal process, once the BSR has achieved its maximum boron loading, NMG is regenerated, typically in a 2-Stage elution/regeneration treatment process employing acid (i.e. sulfuric acid or HCl) for elution of the boron. The polymer-bound cis-diol borate ester complex, described above, is subsequently hydrolyzed and the boron eluted from the resin via an acid rinse (the exact reverse of the loading reaction). This boron liberating hydrolysis is relatively facile at pH less than about 1.0; therefore, relatively high concentrations of acid are required for the complete and rapid elution of the boric acid from BSR. The resin is then treated with base, (i.e. sodium hydroxide) to return the conjugate acid salt of the amino-glucamine functionality, back to its free base form. This neutralization is typically followed by water rinse to remove excess hydroxide subsequent to another boron loading cycle.
Without being bound to theory, I believe that during the first step of regeneration, extra acid is required in the elution step since the NMG functional group is linked to the styrenic backbone through a tertiary nitrogen bridging atom. Up to about 0.9 moles of acid per liter of BSR may be needed, to accommodate the tertiary amine atom's capacity for hydronium ion while it reacts with the acid to form a conjugate acid salt. FIG. 3 illustrates a conjugate acid form of NMG. As a result of the high acid consumption by NMG nitrogens, more acid is required for elution process than would be required to achieve the hydrolysis alone. Only after the bridging nitrogen atoms of the NMG have been protonated, can acid accumulate around the bound cis-diol borate ester complex to an extent great enough to accelerate the boron liberating hydrolysis reaction.
As described above, BSR can be regenerated by hydrolysis of a borate ester complex and eluting the resin with an acid rinse. A typical two-stage process is described in Kunin, R. and Preuss, A. F., 1964, “Characterization of a boron-specific ion exchange resin”, Industrial and Engineering Chemistry: Product, Research and Development 34, pp. 304-306. The Kunin et al. method, which has become an industry standard, employs several bed volumes of water during the regeneration stage, followed by several bed volumes of rinse water.
Since the rate of borate ester hydrolysis increases with lower pH, favorable elution conditions are achieved by slowly passing a small volume (0.30 BV, 2-3 BV/hr) wave of concentrated sulphuric acid through the resin (an amount equivalent to ˜200 mol % of the nitrogen functionality), followed by enough rinse water to displace the acid and boron from the bed. The first 100 mol % forms the conjugate acid salt and the remainder facilitates elution of boron. Following this elution philosophy, essentially complete boron elution can be achieved in a little more than two bed volumes of total eluant. This method differs markedly from the widely applied method of Kunin et al. which utilizes several bed volumes of 1 N sulphuric acid and several bed volumes of rinse water.
One issue with the current two-step regeneration process is that a BSR shrinks and swells depending upon the pH of the aqueous contacting media. Change in volume weakens the macroporous resin, resulting in fractures of the resin. Manufacturers typically deliver BSR in a fully hydrated state, with the weakly basic nitrogen of the NMG functional group is in the “free base” form and the resin is in the shrunken state. However, BSR swells as much as 23% during the acidic boron elution stage and then undergoes shrinkage as the regenerating based neutralizes the conjugate acid. This repeated shrink-swell cycling brings challenges for system designers and can contribute significantly to bringing about a shortened resin life.
In the case of boron removal from seawater reverse osmosis (SWRO) permeate, a second stage is usually required to return the conjugate acid salt of the bound NMG to the free base form. During the second stage, the alkaline regeneration step of the standard regeneration process, addition of base neutralizes all of the conjugate acid sites and the resin shrinks dramatically.
Normally an excess of caustic would be employed to assure complete conversion to the free base form, however, more careful control of the caustic regeneration step is desirable, to return most, but not all of the conjugate acid groups to their corresponding free base form. Additionally, careful control of the caustic regeneration allows a more uniform distribution of converted resin throughout the column, and limits shrinkage of the resin. Standard ion exchange techniques for caustic regeneration will not allow such fine control.
In typical slow-flow regenerating conditions that result in plug-flow regeneration is the concentration of base and acid throughout the column. Hence, at 40% neutralization, 40% of the resin is 100% regenerated while 60% remains un-regenerated. A plug flow of base regeneration solution in a typical regeneration process result in progressive neutralization of the conjugate resin as the alkaline front moves through the resin bed, fully neutralizing the resin behind the front. In order to achieve a high degree of neutralization, excess caustic is typically required and precise titration of the full resin volume to a chosen pH is not possible.
Over-dosage by base in the fully neutralized zone not only results in potentially damaging volume changes in the resin, but also results in “over-shooting” the conjugate acid end-point necessitating rinse-down of the excess caustic, a relatively slow process which usually requires several bed volumes of produced water. In turn, long rinses to achieve effluent pH near 7 or 8 will result in hydroxide loading on the weak base sites as the NMG's amine slightly ionizes water. At pH 7, BSR has been found to be nearly 40% ionized. See Marie-Odile Simonnot, Christophe Castel, Miguel NicolaÏ, Christophe Rosin, Michel Sardin and Henri Jauffret, “Boron removal from drinking water with a boron selective resin: is the treatment really selective?”, Water Research, Volume 34, Issue 1, 1 Jan. 2000, Pages 109-116.
The elimination of the alkaline regeneration altogether would be an approach to limit resin volume change. However, boron capture by BSR's conjugate salt is slow enough that it probably would not be economic to operate. Recepoglu et al., and Nadav, independently investigated the possibility of a single stage elution without alkaline regeneration. See O. Recepoglu and ü. Beker, “A preliminary study on boron removal from Kizildere/Turkey geothermal waste water”, Geothermics, Volume 20, Issues 1-2, 1991, Pages 83-89; and later Nissim Nadav, “Boron removal from seawater reverse osmosis permeate utilizing selective ion exchange resin”, Desalination, Volume 124, Issues 1-3, 1 Nov. 1999, Pages 131-135. Both studies concluded that with low buffering feed such as SWRO permeate, a two stage (acid then base) elution/regeneration protocol similar to the classic conditions of Kunin et al. yields the best performance, while a single stage (acid elution only) process, may be economic in cases having highly buffering feed-stocks.
Simonnot et al. describe one approach to reducing changes in resin volume during regeneration, using an NMG resin that incorporates quaternary ammonium, strongly basic anion exchange sites. See Marie-Odile Simonnot and Stephanie Ouvrard, “Multicomponent Anion Exchange with a Resin Having Weakly and Strongly Basic Groups”, Chemical Engineering Science 60 (2005) 1849-1858. However, use of the quaternary ammonium resin increases the chemical demand of the process over NMG treatment alone.
A related problem associated with the use of BSR columns is that, after regeneration, excess hydroxide bleeds into the product water. Since BSR ionizes significantly at drinking water pH, the large volumes of water required to rinse down excess caustic from freshly regenerated BSR, actually can result in elevated levels of hydroxide bound as anions to the BSR matrix. In the case of boron removal from slightly brackish SWRO permeate, ionically-bound hydroxide ion is liberated via an ion exchange interaction with saline in the permeate (approx. 900 ppm NaCl). This hydroxide ion liberation makes making pH control of the BSR product water problematic.
Small changes in the regenerating base (usually caustic) dose and/or flow rate result in “over shooting” or “under shooting” the target pH of the product water (FIG. 4). In commercial application, the severity of post-regeneration hydroxide bleed, from the resin into the product water stream, even after excessive washing, may necessitate the employment of costly pH control measures. Hydroxide bleed typically persists for several hundred bed volumes after alkaline regeneration and water rinse.
Simonnot et al. suggested that this hydroxide release can be avoided by post regeneration treatment of the resin with a 5 bed volume rinse with 0.5 mol L−1 Na HCO3, followed by more water. Simonnot et al. also noted that at pH 7, BSR was nearly 40% ionized. This means that the N-methyl-D-glucamine functionality retains hydroxide ions in an ion exchange mechanism under drinking water conditions. The relatively high salt content of SWRO permeate (several hundred parts per million chloride) accentuates the problem by initiation of sustained hydroxide bleed, as chloride ion displaces hydroxide ion, after alkaline resin regeneration. Water without the briney components do not produce excessive hydroxide ion bleeding from the resin into the product water stream.